Method and device for cutting metal sheets

ABSTRACT

A metal sheet is cut along a cutting line of given shape by coating the sheet with a layer of insulating material to define an exposed portion of the same shape as the required cutting line; positioning the sheet inside an electrolytic cell having an anode and a cathode; connecting the sheet to the anode; and operating a direct-current generator to generate, between the anode and cathode, a potential difference such as to oxidize the metal of the sheet at the exposed portion.

[0001] The present invention relates to a method of cutting metalsheets.

BACKGROUND OF THE INVENTION

[0002] Metal sheets are normally cut using mechanical, thermal, andchemical cutting techniques.

[0003] Mechanical cutting has the drawback of requiring relativelyhigh-cost dies and subsequent trimming.

[0004] Thermal cutting, normally using a laser device, has the drawbacksof thermally altering the cut material; relatively high operating cost;difficulty in cutting sheets of less than 1 millimeter thickness; andrelatively long cutting time, by depending on the length of the cut.

[0005] Chemical cutting is normally performed using acid, is the mostcommonly used technique, at least for cutting thin sheets, but has thedrawbacks of taking a relatively long time, and producing harmful, toxicresidue which is difficult to dispose of.

SUMMARY OF THE INVENTION

[0006] It is an object of the present invention to provide a method ofcutting metal sheets, designed to eliminate the aforementioneddrawbacks, and which at the same time is cheap and easy to implement.

[0007] According to the present invention, there is provided a method ofcutting a sheet of metal material along at least one given cutting lineto obtain a given finished sheet, the method comprising the steps ofpartly coating the sheet with a layer of electrically insulatingmaterial, so as to define, on the sheet, an exposed portion definingsaid cutting line; positioning said sheet in an electrolytic cellcomprising a direct-current generator, and an anode and a cathodeconnected electrically to said generator; connecting the anode to thecathode by means of an electrolytic conductor; connecting said sheetelectrically to said generator, so that said sheet is part of the anode;activating the generator so as to maintain, between the anode and thecathode, a potential difference such as to oxidize the metal material ofsaid exposed portion; and removing said sheet from said electrolyticcell once the metal material of said exposed portion is oxidized.

[0008] When an electrolytic conductor comprising chloride is used, thechloride may oxidize at the anode to molecular chlorine, which in smallpart is dissolved in the saline solution, and in large part is releasedin the form of gas. Since chlorine is relatively toxic andcontaminating, the method defined above poses the secondary problem ofpreventing chlorine molecules from being released into the surroundingenvironment.

[0009] For this reason, the method defined above preferably comprisesthe further steps of aspirating, inside an absorption chamber, themolecular chlorine produced when said generator is operating; andreducing, inside said absorption chamber, said molecular chlorinesubstantially to chloride by means of an absorption solution.

[0010] The present invention also relates to a device for cutting metalsheets.

[0011] According to the present invention, there is provided a devicefor cutting a sheet of metal material along at least one given cuttingline; the sheet being partly coated with a layer of electricallyinsulating material defining, on the sheet, an exposed portion definingsaid cutting line; and the device comprising an electrolytic cell, inturn, comprising a cathode, an anode, an electrolytic conductorconnecting said cathode and said anode, and supporting means forreceiving said sheet so that the sheet is part of the anode; and adirect-current generator for generating a potential difference betweensaid anode and said cathode to induce oxidation of said metal materialconstituting said exposed portion.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012] A number of non-limiting embodiments of the invention will bedescribed by way of example with reference to the accompanying drawings,in which:

[0013]FIG. 1 shows a longitudinal section of a first embodiment of thedevice according to the present invention;

[0014]FIG. 2 shows a partial cross section of a detail in FIG. 1;

[0015]FIG. 3 shows a schematic, partly sectioned side view of a secondembodiment of the device according to the present invention;

[0016]FIG. 4 shows a plan view, with parts removed for clarity, of adetail in FIG. 1;

[0017]FIG. 5 shows a plan view, with parts removed for clarity, of adetail in FIG. 3;

[0018]FIG. 6 shows a partial cross section of a detail of a thirdembodiment.

DETAILED DESCRIPTION OF THE INVENTION

[0019] Number 1 in FIG. 1 indicates as a whole a device for cutting asheet 2 to obtain a finished sheet 3. Sheet 2 is made of metalmaterial—in this case, iron; is relatively thin—normally 0.01 to 2 mmthick; and comprises two surfaces 4 and 5, of which surface 4 is partlycoated with a layer 6 of electrically insulating material. Morespecifically, layer 6 defines, on surface 4, a coated portion 7 definingan exposed portion 8, in turn defining at least one cutting line 9corresponding, in the example shown, though not necessarily, to theoutline of the required finished sheet 3.

[0020] Device 1 comprises an electrolytic cell 10, in turn comprising anopen-topped tank 11 containing a saline solution 12—in this case, anaqueous sodium chloride solution—which acts as an electrolyticconductor.

[0021] Tank 11 is in the form of an elongated rectangular prism, andcomprises a horizontal bottom wall 13, two vertical walls 14 extendingparallel to a horizontal longitudinal axis 15 of tank 11, and twovertical walls 16 perpendicular to axis 15 and having respectivesubstantially horizontal openings 17, facing and coplanar with eachother, for the passage of sheet 2.

[0022] Each opening 17 is controlled by a respective sealing device 18for preventing saline solution 12 from leaking from tank 11 throughrelative opening 17, and which comprises two horizontal rollers 19 and20 housed, crosswise to axis 15, inside tank 11 and fitted idly to walls14. Rollers 19 and 20 are located respectively under and over the planedefined by openings 17, have respective outer tubular coverings 21 ofelastically deformable material, and are positioned contacting eachother and relative wall 16 with a given pressure and in fluidtightmanner.

[0023] Tank 11 also comprises two gutters 22, each of which is fixed toa respective wall 16, on the outside of cell 10 and just belowrespective opening 17, and provides for collecting any saline solution12 leaking from relative opening 17.

[0024] Tank 11 is fitted inside with a flat, substantially horizontalgrid 23 made of conducting material, e.g. iron or stainless steel 430,and which acts as a cathode. Grid 23 rests on two brackets 24, eachconnected to a respective wall 14, is immersed in saline solution 12,and is connected electrically to a direct-current generator 25 outsidetank 11. By means of a known electric motor (not shown), each bracket 24can be moved, while remaining horizontally coplanar with the otherbracket 24, along a known rack (not shown) extending vertically along arespective wall 14.

[0025] Generator 25 is also connected electrically to a number ofcoplanar rollers 26, which are made of conducting material, e.g.titanium, rotate about respective axes 27 substantially perpendicular toaxis 15, and are located beneath the plane of openings 17 to define asupport, coplanar with openings 17, for sheet 2. Each roller 26 has acylindrical rod 28 connected in rotary manner, at each end, to arelative wall 14 with the interposition of a support 29 of insulatingmaterial, and comprises cylindrical disks 30 coaxial with axis 27 andequally spaced along cylindrical rod 28 to support sheet 2 in a positioncoplanar with openings 17, and with surface 4 facing grid 23. Generator25 is controlled by a control device 31 (shown schematically) which canbe regulated according to the thickness of the material of sheet 2.

[0026] A suction unit 32 is located over cell 10, and comprises a hood33, and a suction device 34 for feeding any gas in hood 33 along aconduit 35 to an absorption chamber 36 defined by a container 37 andfilled with an aqueous basic absorption solution 38 containing, forexample, sodium hydroxide.

[0027] A transfer device 39 is also connected to absorption chamber 36,and comprises a drain pipe 40 controlled by a valve 41 and connectingcontainer 37 to tank 11 to permit flow of absorption solution 38 fromcontainer 37 to tank 11. Valve 41 is controlled by control device 31 asa function of the pH detected by a pH-meter 42, so as to maintain asubstantially constant, substantially neutral pH of saline solution 12.

[0028] In actual use, layer 6 of electrically insulating material isapplied to sheet 2 by means of a known so-called “stencil” method, whichcomprises preparing a known stencil frame (not shown) by depositing aknown photosensitive gel on a metal frame; projecting the outline of therequired finished sheet 3 on to the layer of gel; and washing toeliminate the nonimpressed part of the gel. A known electricallyinsulating ink is sprayed through the known stencil frame (not shown) soformed on sheet 2, so that the exposed portion 8 of surface 4 definesthe outline of finished sheet 3.

[0029] At this point, sheet 2 is treated with ultraviolet (U.V.) orinfrared (I.R.) radiation, depending on the type of electricallyinsulating ink used, to dry the layer 6 of electrically insulatingmaterial defined by the electrically insulating ink; is fed into tank 11through one of openings 17 and between relative rollers 19 and 20; andis positioned resting on rollers 26, with surface 4 facing grid 23.

[0030] At this point, control device 31 activates generator 25, whichgenerates a potential difference between the anode and cathode, so that,at the anode, the iron of exposed portion 8 oxidizes to a ferrous ionaccording to the reaction:

Fe⇄Fe²⁺+2e ⁻ (E⁰=−0.44 V/nhe).

[0031] and dissolves electrolytically in saline solution 12. In thisconnection, it is important to note that the ferrous ion precipitates ashydroxide in a neutral solution of relatively low concentration, ofabout 0.1 M, thus preventing the ferrous ion concentration in salinesolution 12 from increasing to the point of shifting equilibrium of theabove reaction “leftward”, and so preventing an increase in thepotential difference which must be applied for the reaction to proceed.

[0032] If the potential difference generated by generator 25 between thecathode and anode is high enough, the ferrous ion in the solutionoxidizes to a ferric ion according to the reaction:

Fe²⁺⇄Fe³⁺ +e ⁻ (E⁰=+0.77 V/nhe).

[0033] Ferric hydroxide being substantially insoluble in a neutralsolution, such a reaction promotes removal of ferrous ions from thesolution, thus preventing a “leftward” shift in equilibrium of theiron-to-ferrous ion oxidation reaction.

[0034] At the anode, the oxygen in the water may also oxidize, thusproducing gaseous oxygen according to the reaction:

2H₂O⇄O₂+4H⁺+4e ⁻ (E⁰=+1.23 V/nhe).

[0035] This reaction has a standard reduction potential greater thanthat of the ferrous ion, and oxygen oxidation has a relatively highovervoltage on iron and steel electrodes, so that, roughly speaking,predominantly iron oxidizes at the anode.

[0036] At the same time, the hydronium ion in the water is reduced atthe cathode, thus producing gaseous hydrogen according to the reaction:

2H₂O+2e ⁻⇄H₂+2OH⁻ (E⁰=−0.88 V/nhe).

[0037] The ferrous ion may also be reduced at the cathode. On iron andsteel cathodes, however, hydronium ion reduction is predominant,particularly at low Fe²⁺ solution concentrations, and also in view ofthe relatively low overvoltage required to obtain this reaction.Chloride may also oxidize to molecular chlorine according to thereaction:

2Cl⁻⇄Cl₂+2e ⁻ (E⁰=+1.39 V/nhe).

[0038] Part of the molecular chlorine is released as gaseous molecularchlorine, is sucked up by suction unit 32, and is bubbled intoabsorption solution 38. The molecular chlorine in absorption solution 38tends to be reduced according to the reaction:

Cl₂+2e ⁻⇄2Cl⁻

[0039] at the same time oxidizing the oxygen in the water according tothe reaction:

2H₂O⇄O₂+4H⁺+4e ⁻

[0040] Part of the molecular chlorine produced dissolves in salinesolution 12, and can be reduced easily at the cathode according to thereaction:

Cl₂+2e ⁻⇄2Cl⁻

[0041] In this connection, it should be noted that molecular chlorine inwater is in equilibrium with hypochlorous acid according to thereaction:

Cl₂+H₂O⇄HClO+H⁺+Cl⁻ (K_(eq)=2.14×10³)

[0042] Hypochlorous acid is relatively easy to reduce at the cathodeaccording to the reaction:

HClO+H⁺+2e ⁻⇄Cl⁻+H₂O (E⁰=+1.49 V/nhe)

[0043] Once exposed portion 8 is oxidized, thus cutting sheet 2 alongcutting line 9 into finished sheet 3, finished sheet 3 is removed fromcell 10 and washed (in known manner) with a known wash solution ofsodium hydroxide and water of a pH of substantially 14, to remove layer6 of electrically insulating material. In this connection, it should bepointed out that the electrically insulating ink used in the above“stencil” method is often colored, and colors the wash solution withpigments, which can be removed using part of saline solution 12, whichcontains hypochlorite capable of oxidizing and so discoloring thepigments.

[0044] When the pH of saline solution 12 falls below 7, control device31 activates valve 41 as a function of the signals received frompH-meter 42, so that part of absorption solution 38 is fed along pipe 40into saline solution 12 to maintain a substantially constant,substantially neutral pH of saline solution 12.

[0045] Roughly speaking, the above method therefore provides as a wholefor oxidizing and dissolving the iron of exposed portion 8 in solution12 to cut sheet 2 as required. More specifically, it is important tonote that even relatively thin sheets can be cut easily, and that themethod takes a relatively short time, which is independent of the lengthof cutting line 9, and at most depends on the thickness of sheet 2, theway in which the electric field is applied, the type of electrolyticconductor, the type of metal sheet 2 is made of, and the distancebetween the cathode and anode.

[0046] In the FIG. 3 embodiment, electrolytic cell 10 forms part of anautomated assembly 43 for producing finished sheets of given shape. Inthis case, sheet 2 forms part of a strip 44, which, in use, is drawn offan unwinding reel 45 and, once the finished sheet 3 is formed anddetached from strip 44, is wound on to a winding reel 46 powered by anelectric motor 47 under the control of control device 31. Morespecifically, electric motor 47 may, for example, be a direct-currentmotor with an encoder, an asynchronous motor with an inverter, or abrushless motor with a resolver.

[0047] Strip 44 is unwound off reel 45 and fed through a known wash unit48 (shown schematically) and a known degreasing unit (not shown) to aknown stencil unit 49 (shown schematically), where, by means of thestencil method described previously, layer 6 of electrically insulatingmaterial is applied successively to each sheet 2 defining strip 44, andis then dried, depending on the material of layer 6, by known infraredor ultraviolet lamps 50 (shown schematically) forming part of a knownfollow-up drying unit 51 (shown schematically).

[0048] It should be pointed out that, in this case, the electricallyinsulating ink is deposited so that, on each sheet 2 forming strip 44,portion 8 left exposed by the electrically insulating ink does notdefine the whole outline of finished sheet 3, but comprises at least onebreak—in the example shown, three—of a few, e.g. two, tenths of amillimeter, so that, once exposed portion 8 is oxidized, sheet 2, asopposed to being detached completely from strip 44, remains attached toit by microjoints 52.

[0049] Each sheet 2 on strip 44 is then fed into cell 10 through opening17, between rollers 19 and 20, and along rollers 26, and, once exposedportion 8 is oxidized, remains integral with strip 44 when fed towardswinding reel 46. Downstream from cell 10, each sheet 2 on strip 44 isfed through a known stripping unit 53 (shown schematically), in whichlayer 6 is washed off in a sodium hydroxide and water wash solution witha pH of substantially 14, so as to obtain a finished sheet 3 partlyattached to strip 44 by microjoints 52.

[0050] The resulting strip 44 is then washed in a known wash unit 54(shown schematically) and dried in a known drying unit (not shown)before being fed to a known parting unit 55 (shown schematically) whereelectric discharges are applied to eliminate microjoints 52.

[0051] Alternatively, microjoints 52 can be severed mechanically by anoperator to detach finished sheets 3 from strip 44.

[0052] It should be pointed out that, by means of control device 31, itis possible to adjust the parameters of the electric field inelectrolytic cell 10; control and adjust the pH of the saline solutionby operating valve 41; adjust the hold time of sheets 2 inside the aboveoperating units by controlling operation of electric motor 47; andadjust the anode-cathode distance by operating said known electric motor(not shown).

[0053] In a variation not shown, as opposed to the “stencil” method,layer 6 of insulating material is deposited on surface 4 using a knownso-called “dry film” method, whereby a light-sensitive film is depositedon the whole of surface 4, and on which the outline of the requiredfinished sheet 3 is projected in negative. At this point, sheet 2 iswashed with acid to remove the nonimpressed part of the film bycorrosion and form, on sheet 2, an exposed portion 8 reproducing theshape of cutting line 9.

[0054] In a further variation not shown, instead of or in addition tosodium chloride, solution 12 comprises sodium sulfate and/or sodiumnitrate and/or other water-soluble salt. In which case, device 1 andoperation of the device are substantially the same as described above,except that, in the absence of chloride, and therefore of the chlorideoxidation and associated reactions, absorption chamber 36 of device 1may be dispensed with.

[0055] In a further variation not shown, sheet 2 is of nonferrous metalmaterial, e.g. copper or copper alloy. In which case, device 1 and themethod only differ from those described above by the nonferrous metal,as opposed to iron, oxidizing at the anode.

[0056] In the FIG. 6 variation, layer 6 of electrically insulatingmaterial is deposited on both surfaces 4 and 5. In which case, device 1and the method only differ from those described by the “stencil” methodor “dry film” method being applied to both surfaces 4 and 5 of sheet 2.

1) A method of cutting a sheet (2) of metal material along at least onegiven cutting line (9) to obtain a given finished sheet (3), the methodcomprising the steps of partly coating the sheet (2) with a layer (6) ofelectrically insulating material, so as to define, on the sheet (2), anexposed portion (8) defining said cutting line (9); positioning saidsheet (2) in an electrolytic cell (10) comprising a direct-currentgenerator (25), and an anode and a cathode connected electrically tosaid generator (25); connecting the anode to the cathode by means of anelectrolytic conductor (12); connecting said sheet (2) electrically tosaid generator (25), so that said sheet (2) is part of the anode;activating the generator (25) so as to maintain, between the anode andthe cathode, a potential difference such as to oxidize the metalmaterial of said exposed portion (8); and removing said sheet (2) fromsaid electrolytic cell (10) once the metal material of said exposedportion (8) is oxidized. 2) A method as claimed in claim 1, wherein saidelectrolytic cell (10) comprises a saline solution (12), in which saidcathode and said anode are immersed; said saline solution (12)comprising said electrolytic conductor (12). 3) A method as claimed inclaim 2, wherein said saline solution (12) comprises chloride. 4) Amethod as claimed in claim 3, and comprising the further steps ofaspirating, inside an absorption chamber (36), the molecular chlorineproduced when said generator (25) is operating; and reducing, insidesaid absorption chamber (36), said molecular chlorine substantially tochloride by means of an absorption solution (38). 5) A method as claimedin claim 4, and comprising the further step of maintaining asubstantially constant pH of said saline solution (12) by feeding partof said absorption solution (38) into the saline solution (12) in saidelectrolytic cell (10). 6) A method as claimed in claim 2, wherein saidsaline solution (12) selectively comprises one of the following:sulfate, nitrate, a combination of these. 7) A method as claimed inclaim 1, wherein said cathode comprises a grid (23) made of electricallyconducting material. 8) A method as claimed in claim 7, wherein saidelectrically conducting material comprises iron. 9) A method as claimedin claim 7, wherein said sheet (2) is placed inside said electrolyticcell (10) in a position substantially parallel to said grid (23). 10) Amethod as claimed in claim 9, wherein said sheet (2) has two oppositesurfaces (4, 5), only one (4) of which is coated with said layer (6) ofelectrically insulating material; said sheet (2) being positioned insidesaid electrolytic cell (10) with said coated surface (4) facing saidgrid (23). 11) A method as claimed in claim 1, wherein said sheet (2)has two opposite surfaces (4, 5), both of which are coated with saidlayer (6) of electrically insulating material. 12) A method as claimedin claim 9, wherein said grid (23) is larger than said sheet (2). 13) Amethod as claimed in claim 1, wherein said sheet (2) is a single sheet.14) A method as claimed in claim 1, wherein said sheet (2) forms part ofa strip (44), which is fed through said electrolytic cell (10). 15) Amethod as claimed in claim 13, wherein said cutting line (9) has atleast one break defining a respective microjoint (52) between therelative finished sheet (3) and said strip (44). 16) A method as claimedin claim 14, and comprising the step of mechanically severing saidmicrojoint (52) to detach each said finished sheet (3) from said strip(44). 17) A method as claimed in claim 1, and comprising the step ofwashing said sheet (2) before coating it with said layer (6) ofelectrically insulating material. 18) A method as claimed in claim 1,and comprising the step of removing said layer (6) of insulatingmaterial from said finished sheet (3). 19) A method as claimed in claim18, wherein said removing step comprises washing said finished sheet (3)with a substantially basic wash solution. 20) A method as claimed inclaim 19, and comprising the step of feeding part of said salinesolution (12) into said wash solution. 21) A device for cutting a sheet(2) of metal material along at least one given cutting line (9); thesheet (2) being partly coated with a layer (6) of electricallyinsulating material defining, on the sheet (2), an exposed portion (8)defining said cutting line (9); and the device (1) comprising anelectrolytic cell (10), in turn, comprising a cathode, an anode, anelectrolytic conductor (12) connecting said cathode and said anode, andsupporting means (26) for receiving said sheet (2) so that the sheet (2)is part of the anode; and a direct-current generator (25) for generatinga potential difference between said anode and said cathode to induceoxidation of said metal material constituting said exposed portion (8).22) A device as claimed in claim 21, wherein said electrolytic cell (10)comprises a saline solution (12), in which said cathode and said anodeare immersed; said saline solution (12) comprising said electrolyticconductor (12). 23) A device as claimed in claim 21, wherein said salinesolution (12) comprises chloride. 24) A device as claimed in claim 25,and comprising an absorption chamber (36) containing an absorptionsolution (38), and suction means (32) for aspirating gaseous molecularchlorine from said electrolytic cell (10) to said absorption chamber(36); said absorption solution (38) reducing said molecular chlorine tochloride. 25) A device as claimed in claim 24, and comprising firsttransfer means (39) for transferring part of the absorption solution(38) from said absorption chamber (36) to said electrolytic cell (10).26) A device as claimed in claim 25, and comprising a control device(31), and at least one measuring means (42) for measuring the pH of saidsaline solution (12) and connected to said control device (31); saidcontrol device (31) controlling said first transfer means (39) tomaintain a substantially constant pH of said saline solution (12). 27) Adevice as claimed in claim 22, wherein said saline solution (12)comprises sulfate. 28) A device as claimed in claim 21, wherein saidcathode comprises a grid (23) made of conducting material. 29) A deviceas claimed in claim 28, wherein said conducting material comprises iron.30) A device as claimed in claim 21, and comprising a stripping unit(53) for removing said insulating material from said finished sheet (3).31) A device as claimed in claim 30, wherein said stripping unit (53)comprises a substantially basic wash solution. 32) A device as claimedin claim 21, wherein said sheet (2) forms part of a strip (44); thedevice (1) comprising feed means (47) for feeding said strip throughsaid electrolytic cell (10). 33) A device as claimed in claim 32,wherein said feed means (47) are connected electrically to said controldevice (31); said control device (31) operating said feed means insteps. 34) A device as claimed in claim 32, and comprising a partingunit (55) for detaching said finished sheet (3) from said strip (44).35) A device as claimed in claim 21, wherein said electrolytic cell (10)comprises an open-topped tank (11) containing the saline solution (12),and having two parallel vertical walls (16) having respectivesubstantially horizontal openings (17) facing and coplanar with eachother for the passage of the sheet (2). 36) A device as claimed in claim35, wherein, adjacent to each opening (17), the electrolytic cell (10)comprises sealing means (18) for said saline solution (12). 37) A deviceas claimed in claim 36, wherein said sealing means (18) comprise twohorizontal rollers (19, 20) located respectively below and above a planedefined by said openings (17); each said roller (19, 20) resilientlycontacting in fluidtight manner the other roller (20, 19) and saidvertical wall (16) of the relative said opening (17). 38) A device asclaimed in claim 24, and comprising movable supports (24) on which saidgrid (23) rests; the movement of said movable supports (24) beingcontrolled by said control device (31).